A new application of the ’pore filling’ concept yielded high-performance composite membranes for the selective pervaporation (PV) separation of organic mixtures, Asymmetric polyacrylonitrile (PAN) membranes (average pore sizes of 7 or 12nm) were used as matrix for polymeric PV separation phases which were in situ prepared by heterogeneous photoinitiated graft copolymerization. By this means, defect-free and stable layers were synthesized from various moderately hydrophilic (meth)acrylates, e.g. poly(ethylene glycol) (meth)acrylates. The impact of (meth)acrylate side-group functionality (hydrophilicity, size) and preparation parameters (monomer concentration, UV irradiation time) was analyzed using PV methanol removal from less polar hydrocarbons (cyclohexane or MTBE) as example. High selectivities (alpha(methanol/cyclohexane) less than or equal to 2000) and extraordinarily high permeate fluxes (J less than or equal to 8 kg/m(2) h) were achieved. Major reasons for the excellent performance were the small effective PV barrier thickness (< 1 mu m; determined by the thickness of the selective layer of the matrix membrane) and the fixation of the graft polymer (covalent anchoring, suppressed swelling) in a matrix (PAN) which is not swollen under PV conditions, Due to the functionalization principle, a great variety of thin-layer composite membranes can be prepared, ’tuning’ the PV selectivity by functionality of the graft polymer, thus providing possibilities to efficiently solve various PV separation problems.